There is NO dam on the Wisconsin River in Sauk City. It's in Prairie du Sac, the northern "suburb" of Sauk City.
Coming in on Hwy 12 from the south, turn right (north) at the interchange at the end of the Wisconsin River bridge onto Water St. Continue north on Water St. until you come to a 4-way stop at Hwy. 60 and Prairie St. in Prairie du Sac. Continue north (straight ahead) until you come to VFW Drive. That takes you to the nearest (and best) boat launch below the dam on the Wisconsin River that forms Lake Wisconsin.
The 2 villages (with separate governments, maintenance depts., etc.) are the result of a religious dispute arising in the 1800's.
The LakeLinks site "Wisconsin River Prairie du Sac" is where most of the posting about fishing below the dam is done.....